The InfiniBand (IB) Architecture defines a computer networking architecture that allows end nodes to communicate with one another via a fabric or subnetwork (subnet) comprising one or more switches. A full definition of the IB Architecture may be found in the InfiniBand Architecture Specification Volumes 1 and 2, available from the InfiniBand Trade Association (www.infinibandta.org), which is hereby incorporated by reference.
The IB specification does not define any inherent restrictions on how to construct subnet topologies in terms of interconnecting switches, apart from the main requirement that the resulting routes between end-nodes in the subnet must be free of potential deadlocks. This implies that a large number of configurations in terms of inter-switch connectivity may be legal according to the formal IB specification. However, such configurations may be sub-optimal or inadequately tested by a system vendor.
In addition, since the IB specification does not impose any real restrictions in terms of switch topology, a standard subnet manager implementation typically by default does not enforce any such restrictions. Indeed, the subnet manager may not even have support for explicit instructions to impose such restrictions. Consequently, if it is desired to select a routing through a particular set of links, it may not be possible to prevent the subnet manager from taking other (legal) switch-to-switch links into consideration within the routing logic.